This invention relates to a method of making an integrated circuit resistor element on a semiconductor body and, more particularly, to a method of making an ion implanted or diffused integrated circuit resistor on a silicon substrate whose resistance value accurately corresponds to its nominal design value.
The conventional technique of forming an integrated circuit resistor is by ion implanting or diffusing a predetermined area of a semiconductor substrate, typically, rectangular in shape forming a resistor bar having a known width W and sheet resistance .rho..sub.s, and placing metal contacts contacting the bar at a known separation L which determines the length of the resistor. From the known design values of .rho..sub.s, W and L the design resistance value R is deduced by the approximate relationship R=.rho..sub.s L/W. However, due to process tolerances and dimension tolerances, the actual or measured value may significantly differ from the design value resulting in poor or erroneous performance of the circuits utilizing the resistor element.
To briefly elaborate on the above-mentioned deleterious process and dimension tolerances, in the fabrication of the integrated circuit resistor elements in conjunction with other passive elements and active elements which together constitute the integrated circuit, numerous processing steps such as ion implantation/diffusion, epitaxial growth, metallization, etc., coupled with corresponding lithographic steps involving a large number of masks are performed. For example, in very large scale integration technologies, some 10 different masks are required for an insulated gate field effect transistor and approximately 15 masks are used for a bipolar transistor. It is well known that when the semiconductor body is subjected to an ion diffusion step by depositing dopant species in a designated area followed by thermal drive-in to form the resistor bar generally simultaneously with other elements (e.g., the base of a NPN transistor) of the integrated circuit, the dopant profile will not be uniform due to inherent variations in the dopant species concentration, temperature, etc. Similar variation occurs when the resistor bar is formed by ion implantation. As a result, the sheet resistance of the resistor bar differs from the design value. Likewise, the various masks used in the fabrication of the resistor element vary from the specification established by the designer, as a consequence of undesirable effects such as over-exposure or under-exposure of the organic (photoresist) layers which may occur during the fabrication of the masks. Also, even if the width of the mask is equivalent to the desired nominal width W of the resistor bar, any over-exposure or under-exposure of the photoresist layer on the wafer and any overetching or underetching of the insulating layer will result in the ion diffused or implanted region being too wide or too narrow as compared to the nominal value.
A more complete expression for the resistance of the resistor than the one discussed above which takes into account contact resistance and current crowding resistance can be written as: EQU R=R.sub.b +R.sub.cc +R.sub.c ( 1)
where R.sub.b designates the resistance of the body of the resistor having a constant width W, R.sub.cc resistance due to current crowding and R.sub.c is the contact resistance associated with the interface between the resistor bar and the metal contact. To explain the various terms of equation (1), reference is made to FIG. 1 wherein is shown in top view a rectangular resistor bar generally designated by numeral 10 of width W having two metal contacts 11 and 12 formed at a separation L corresponding to the nominal design length of the resistor which is obtained from the contact level mask. The metal contacts 11 and 12 for the resistor shown do not extend over the entire width W of the resistor bar 10. Consequently, when the contacts 11 and 12 are maintained at different electrical potentials, since the electric charges tend to take the path of least resistance, the resistor bar, instead of having a rectangular section of length L and width W will have two current crowding sections 13 and 14 and a body portion 15. As illustrated in FIG. 1, the current crowding sections 13 and 14 have a varying width and a relatively small length both contributing a resistance R.sub.cc. The body portion 15 is rectangular in shape having a length L.sub.b and width W contributing a resistance R.sub.b =.rho..sub.s L.sub.b /W. The resistance R.sub.c is due to contact resistance associated with the interfaces between the resistor bar 10 and the metal contacts 11 and 12.
As a result of the above variation .DELTA..rho..sub.s in sheet resistance and the variation .DELTA.W in the resistor width, the actual resistance may be significantly different from the design value unless the spacing between the contacts 11 and 12 (i.e., the length L of the resistor) is appropriately adjusted. For example, when the net contribution to the resistance due to the process and image tolerances is positive, the spacing between the contacts 11 and 12 may be required to be shortened by a corresponding amount .DELTA.L as shown in FIG. 2 to compensate for the positive contribution. Likewise, when the net contribution to the resistance due to the various tolerances is negative, the spacing L may be required to be increased by .DELTA.L as shown in FIG. 3 in order to accurately compensate for the negative contribution and match the actual resistance of the resistor element with its nominal design value.
In the prior art process of forming integrated circuit resistor elements, since the contact level mask is designed and fabricated at the outset in accordance with the particular integrated circuit chip design, it is impossible to correct for the above errors by appropriately adjusting the separation L between the contact openings 11 and 12 for each resistor element on the chip. The only way it is possible to obtain a resistor whose measured value closely matches the nominal design value is by exercising stringent controls over the ion diffusion/implantation process step, over image tolerances, over the relevant etching steps, etc. Despite such tight controls, deviations in the order of 15-20% of the actual resistance from the design value are unavoidable.
Accordingly, it is an object of this invention to provide a method of forming precision resistors on a semiconductor body.
It is another object of this invention to provide a method of forming integrated circuit resistor whose resistance value accurately corresponds to its nominal design value by correcting during its fabrication for variation inherently introduced due to process variations and image tolerances.
It is yet another object of this invention to provide a method of making highly accurate resistors integrated with other passive and active elements of an integrated circuit by dynamically making individual photo process adjustments for the resistors' contact locations.